A large part of the world still lacks safe drinking water. The world health organization (WHO) estimates that roughly 1.1 billion people in developing countries do not have access to potable drinking water, and 2 million die every year due to drinking water-related diseases (WHO/UNICEF, 2006). Children are more vulnerable and every day the world loses roughly 3900 children below age 5 (WHO/UNICEF, 2004). A third of the people living in rural areas, slums and poor suburbs in developing countries use streams, ponds, rainwater from roofs, and poorly constructed wells that can be, contaminated with pathogens. The majority of contamination originates from human and animal fecal matter that, due to poor sanitation, ends up in streams and wells. These problems are acute in sub-Saharan Africa, and some countries in Oceania, eastern and southern Asia (Dejung et al., 2007; Gadgil, 1998). FIG. 1 shows world coverage of improved (conventional) drinking water systems; in Africa less than 50% of people have access to improved drinking water facilities. Forty two percent of the sub-Saharan African population use unimproved water sources, and are the largest population without access to improved drinking water sources, estimated to be around 528 million people, lives in South and East Asia (WHO/UNICEF, 2006).
There are six main categories of drinking water related diseases: —diarrhea; caused by some microbial pathogens, —ascaris, —dracunculiasis, —hookworm and other worms, —schistosomiasis, and —tracoma (Gadgil, 1998). These diseases can lead to disability, morbidity, and death. Also, chemical contamination, in particular arsenic and fluoride naturally present in some soils, is fairly common in developing countries. Lack of potable water is also one of the hindrances to economic development. Sickness in adults who are breadwinners affects the whole family in households. Diarrheal diseases in children reduce food absorption which in turn causes malnutrition, and impairs physical and mental development. Women in affected regions have to walk long distances to find potable water and spend time caring for sick children. Beside loss of life, another major consequence of lack of potable water is lost productivity due to sickness (Gadgil, 1998).
Solar disinfection debuted when Acra et al. (1984) observed inactivation of enteric bacteria due to solar radiation. Solar disinfection health benefits were emphasized when a study on Kenyan children under 5 years old found that children who drank water filled in PET plastic bottles exposed to sunlight for a day (SODIS: solar disinfection), showed a 16-24% reduction in incidence of all diarrhea episodes and a 86% reduction of cholera during an outbreak (Meierhofer, 2006). In 2006 a survey showed that 2 million people in poor communities in 20 countries use SODIS and as a result diarrhea incidence has been reduced by 30 to 70% (Meierhofer, 2006). This low cost technology, SODIS, consists of exposing a plastic bottle filled with water to sunlight of at least 500 W/m2 for 5 hours (Oates et al., 2003). SODIS has been demonstrated as a system applicable in regions between latitude 35° North and 35° South, with at least a six-hour exposure to get a 3-4 log10 (99.9-99.99%) reduction in E. coli, Vibrio cholera, Salmonella spp, Shigella spp, Rotavirus, Giardia and a 2-3 log10 (99-99.9%) reduction in Cryptosporidium spp. (Meierhofer, 2006).
Disinfection Using Ultraviolet Radiation
The fundamental behavior of UV radiation as a disinfectant has been examined in detail. Some biomolecules within microorganisms, such as nucleic acids and proteins, have high absorption in the UV region of the electromagnetic spectrum and may undergo (mutagenic) photochemical alterations when exposed to UV (Jagger, 1967). The most common form of UV-induced damage within nucleic acids is photodimerization of adjacent pyrimidine bases. Another form of UV induced damage, apart from mutagenic effects to DNA, is loss in activity in some cell proteins; proteins with high amounts of tryptophan, tyrosine, phenylalanine amino acids are prone to UV damage (Blatchley and Peel, 2001). It is believed that mutagenic effects prevent transcription and the cell cannot replicate. If a cell cannot multiply, it can no longer cause infection (EPA, 2006).
Solar UV Disinfection
The antimicrobial effect of solar UV depends on the spectrum of radiation reaching the Earth's surface. The “near UV” range (290-400 nm) represents wavelengths of solar radiation that can reach the Earth. Peak DNA absorbance generally lies between 250 and 260 nm. However, radiation in this wavelength range is not included in the solar UV spectrum at the Earth's surface because of absorbance by atmospheric constituents. In addition to photodimerization, it has also been suggested that the damage occurs at the cell's chromophores and their prosthetic groups, also called endogeneous photosensitizers (FAD, NAD, heme, quinons, porphyrins, Fe—S cores) (Dejung et al., 2007). These chromophores are known to absorb radiation in the near-UV and above 400 nm. The shift of chromophores to other excitation states releases some energy and due to absence of energy acceptors, absorbance of these photons can lead to conformational changes in the chromophores, thus causing a biological activity loss. On the other hand, oxygen can act as energy acceptor and generate radicals, also known as reactive oxygen species, which are capable of causing damage to the cell membrane (Dejung et al., 2007). Reactive oxygen species also damage DNA by strand breaks and base changes. Also, in the presence of solar UV, exogenous photosensitizers such as humic substances react with oxygen and generate highly reactive oxidants such as hydrogen peroxide (H2O2), singlet oxygen and superoxides, which may display antimicrobial behavior (Kehoe et al., 2004). Exposure of E. coli to near-UV radiation has also been linked to impairment of membrane transport and the catalase enzyme system, leading to inactivation (Vidal and Diaz, 2000).
Action Spectra
An action spectrum of a microorganism provides a quantitative measure of its relative response at different wavelengths. For polychromatic UV sources, it is important to know what wavelengths are involved in a given action and how much effect each wavelength induces compared to other wavelengths. The action spectrum can also be described as the reciprocal of the dose required for a given effect versus wavelength (Jagger, 1967). The reciprocal of the dose versus wavelength shows that wavelengths that require the lowest dose for a given effect are the most efficient. As an example, FIG. 2 shows the relative germicidal effectiveness for inactivating E. coli. FIG. 2 shows that to inactivate E. coli using wavelength in near UV, for example using 3000 Å (300 nanometers; nm), the dose required is roughly 10 times the dose required at around 2600 Å (260 nm), it jumps to 1000 times at around 3400 Å (340 nm) and 10000 times at 4000 Å (400 nm).
Microbial Cell Repair Mechanisms
Some microorganisms are equipped with DNA repair mechanisms to respond to UV damage. There are four main enzymatic repair mechanisms; one is the light-dependent repair mechanism and three are dark-repair mechanisms. Light-dependent repair reverses UV-induced damage for pyrimidine dimers with the help of photolyases (photoreactivating enzymes). These enzymes work in presence of UV and visible light (300-600 nm). Of the dark-repair mechanisms, the most commonly observed repair is nucleotide excision repair (NER) and is known to be slow. NER repair occurs on pyrimidine dimers and other lesions not caused by UV. Some E. coli cell proteins that participate in NER include DNA polymerase I, ligase, Uvr A, Uvr B, Uvr C and Uvr D. A second dark-repair process is known as ultraviolet excision repair. It occurs in only pyrimidine dimmers caused by UV radiation and is relatively faster than NER. The third dark-repair mechanism involves an enzyme known as glycosylase to repair pyrimidine dimers (Blatchley and Peel, 2001).
Solar Radiation
Solar radiation is electromagnetic energy that originates from the Sun. The outer layer surface of the Sun is made up of cooler gases which are strong energy absorbers. This layer has properties similar to a blackbody, with a temperature estimated to be around 5777 K. The energy absorbed and reemitted by the black body is the radiation that is emitted by the Sun. The Earth, which is 1.5*1011 m away of the Sun, receives roughly 1.5×1018 kW·h per year of solar radiation energy, or approximately 28,000 times the world energy consumption per year (Duffie and Beckman, 1991).
Due to eccentricity of the Earth's orbit around the Sun, the energy that is received at the outer edge of the Earth's atmosphere (also known as the solar constant) varies daily, with a yearly average energy of 1367 W/m2 at an average solar-earth distance of 93 million miles. This yearly variation solar constant variation is shown in FIG. 3.
Solar radiation received at the surface of Earth varies with latitude, longitude and ordinal date of the year. In addition, due to Earth's rotation, for an observer on Earth's surface the Sun sweeps an arc from sunrise in the morning to sunset in the evening with high energy at noon when the Sun is at zenith. The more you move away from the equator toward the South or North Pole, the more solar rays become inclined and have to travel a longer distance through Earth's atmosphere, and hence more attenuation they experience. The angle at which the solar rays strike a horizontal surface on Earth surface is called the incidence angle. Solar radiation also varies according to altitude and local climate. The spatial distribution of solar radiation is illustrated in FIG. 4. On this graph the world solar radiation distribution is expressed in Kw·h per square meter per year. The highest energy is located in regions near the tropics in regions depicted with dark red color and lowest as you approach the poles in regions marked with yellow color. Cloud cover is also important to attenuation of solar radiation. As an example, near the equator solar rays tend to be perpendicular to horizontal surfaces, but due to the presence of clouds formed by equatorial rain forests and movement of winds, solar radiation is often less intense than radiation observed in the deserts (e.g. Sahara and Kalahari) near the tropics in Africa (FIG. 4).
Solar Radiation Collection
Background on Solar Radiation Collection Technologies
Solar energy collectors are devices that collect, concentrate and transfer energy to a fluid or convert it to electric current. These collectors can be classified with respect to whether they are concentrating or non-concentrating. They can also be classified with respect to whether they are stationary or tracking. In the category of stationary collectors, two types of collectors are mostly used in solar energy: Flat plate collectors (FPC) and Stationary compound parabolic collectors (CPC).
In the category of tracking collectors, there are many geometries that are in use and among them the basic geometries include: Parabolic trough collector (PTC), Cylindrical trough collector (CTC), and Linear Fresnel reflector (LFR).
Types of Collectors
Flat Plate Collectors (FPC)
Flat plate collectors are more economical than most other collectors (Kalogirou, 2004). They are mostly used in low temperature fluid heating applications. In this device, pipes that carry the fluid run on a flat absorber (e.g. sheet metal) with insulation covers on sides and bottom, and glazing glass at the top (FIG. 2.7). For heating applications the absorber sheet metal is coated with a black color and the glass cover serves as a protection against convection and reradiation losses. The glass cover transmits short wavelength radiation from the sun but it is opaque to long-wave thermal radiation from the absorber plate. Flat plate collectors are mostly static and they are oriented toward the equator; oriented south in the northern hemisphere and oriented north in southern hemisphere (Kalogirou, 2004).
Compound Parabolic Concentrator (CPC)
The CPC is a unit of two parabolic sections facing each other. The non-tracking CPC was originally designed for solar thermal collection purposes: CPCs include a reflector system that directs solar radiation to the absorber without tracking the sun (independent on incidence angle). Using one or multiple internal reflections, the CPC allows collection of nearly 70% of global solar radiation without a tracking system. This system also is able to collect both diffuse and beam radiation. Diffuse radiation represents roughly 50% of total solar UV radiation (Vidal and Diaz, 2000). FIG. 5 shows the geometric characteristics of a CPC; it is made up two reflective side walls and a receiver pipe or plane is fixed at the bottom where those walls meet. The angle between the diagonals is the acceptance angle (θ). The acceptance angle is also defined as the angle through which all radiation with incidence angle smaller than acceptance angle strikes the absorber. The CPC takes the radiation received at the aperture area and concentrates it to a smaller collection area; the concentration ratio (C) is defined as following in Equation 2.1:C=a/a′=1/sin θ.  Equation 2.1Where a is aperture size, a′ is the receiver size, and θ is the acceptance half-angle.
The absorber area can have various configurations depending upon use; it can be a flat area or a tube as shown in FIG. 5. A CPC longitudinal axis can be oriented east-west or north-south. When it is oriented east-west, it can collect radiation along daily solar movement without tracking. The transversal axis is tilted to the south in northern hemisphere and tilted north in southern hemisphere with little seasonal adjustment. For stationary CPCs in this mode, the acceptance angle should cover declination of the Sun from summer to winter solstices (47° equivalent to 23.5° for each solstice). When the CPC longitudinal axis is oriented in the north-south direction, it should be tilted to track the Sun, such that the solar incidence angle remains within the acceptance angle.
Tracking Collectors
Almost all tracking collectors are also concentrators; a tracking system follows the Sun's movement in the sky. It should maintain its ability to track the Sun during intermittent cloud cover and clear sky, protect the collector against severe environmental conditions such as wind, overheating and alarm in case of failure. The most popular types of tracking concentrators are described below.
Parabolic Collector
A reflector of parabolic shape is mounted in the background of the evacuated pipe. The reflector is positioned in the direction of the Sun in such a way that the solar rays hit perpendicularly to the plane of the reflector and get reflected to a focal point or line. Most parabolic collectors, except compound parabolic collectors, require a Sun tracking system that allows it to rotate around an axis oriented north-south or east-west or both. This type can achieve very high concentration since a large area can be concentrated on a very small area.
Parabolic collectors can be categorized in two major types; parabolic trough and parabolic dish (circular). For a parabolic trough the energy is concentrated to a focal line while a parabolic dish directs collected radiation to a focal point. The size of the tube or collection point depends on the size of concentrated Sun's image and manufacturing tolerances of the collector. A parabolic trough with an axis facing north-south collects more solar energy than an east-west oriented system in summer, while east-west system collects more energy during winter. However, over a year, the energy collected is slightly higher for a system with a north-south orientation. Also, parabolic dishes have a double-axis tracking system (Kalogirou, 2004).
Linear Fresnel Reflector (LFR)
This collector is a composite of individual mirror strips which concentrate radiation on a fixed-point receiver. This system has an advantage in manufacturing as the reflecting strips are flat and do not require precision curving compared to parabolic collectors. In addition each strip can be maintained separately. The receiver is usually static and mirror strips require a tracking system to direct radiation to the receiver.
Collector Materials
Reflecting materials make it possible to deviate and concentrate solar radiation intensity by collecting solar rays on a certain area and transferring them to a smaller area. The ideal or maximum concentration gained equals the ratio of the two areas. The ideal concentration would correspond to a system that reflects, transmits and collects 100% of energy falling on the collector, and in reality such a system is not feasible, because of imperfections in manufacturing and wear and tear from UV (this will be discussed later in reflecting materials section). Transmitting materials are used for covers of the system and tubes, allowing radiation to be transmitted to the target fluid or the absorber.
The integrated solar reflectance at incidence angle θ (Rsol(θ)) is defined as (Equation 2.2) (Nostell, 2000).
                              Rsol          ⁡                      (            θ            )                          =                                            ∫              λ1              λ2                        ⁢                                          R                ⁡                                  (                                      λ                    ,                    θ                                    )                                            *                              Ssol                ⁡                                  (                  λ                  )                                            *                                                          ⁢                              ⅆ                λ                                                                        ∫              λl              λ2                        ⁢                                          Ssol                ⁡                                  (                  λ                  )                                            *                                                          ⁢                              ⅆ                λ                                                                        Equation        ⁢                                  ⁢        2.2            Where R(λ, θ) is the wavelength dependent reflectivity at angle θ, Ssol (λ) is the spectral solar irradiance and the limits on integration are the boundaries of UV wavelength. Similarly, the replacement of R by T allows calculation of the integrated solar transmittance (Equation 2.3) (Nostell, 2000).
                              Tsol          ⁡                      (            θ            )                          =                                            ∫              λ1              λ2                        ⁢                                          T                ⁡                                  (                                      λ                    ,                    θ                                    )                                            *                              Ssol                ⁡                                  (                  λ                  )                                            *                                                          ⁢                              ⅆ                λ                                                                        ∫              λl              λ2                        ⁢                                          Ssol                ⁡                                  (                  λ                  )                                            *                                                          ⁢                              ⅆ                λ                                                                        Equation        ⁢                                  ⁢        2.3            For reflectors a portion of energy received is lost through wavelength-dependent absorbance (Asol(θ)) (Equation 2.4). The integrated absorbance equals 1 minus integrated reflectance (Equation 2.5) (Nostell, 2000).
                              Asol          ⁡                      (            θ            )                          =                                            ∫              λ1              λ2                        ⁢                                          (                                  1                  -                                      R                    ⁡                                          (                                              λ                        ,                        θ                                            )                                                                      )                            *                              Ssol                ⁡                                  (                  λ                  )                                            *                                                          ⁢                              ⅆ                λ                                                                        ∫              λl              λ2                        ⁢                                          Ssol                ⁡                                  (                  λ                  )                                            *                                                          ⁢                              ⅆ                λ                                                                        Equation        ⁢                                  ⁢        2.4                                          A          ⁡                      (            λ            )                          =                  1          -                      R            ⁡                          (              λ              )                                                          Equation        ⁢                                  ⁢        2.5            The total integrated reflectance (Equation 2.6) is equal to the sum at all incidence angles; in the equation below W (θ) is the angular weight function; the limits are angles of incidence which vary from 0° to 90°.
                    Rsol        =                                            ∫              0                              90                ⁢                °                                      ⁢                                          Rsol                ⁡                                  (                  θ                  )                                            *                              W                ⁡                                  (                  θ                  )                                            *                                                          ⁢                              ⅆ                θ                                                                        ∫              0                              90                ⁢                °                                      ⁢                                          W                ⁡                                  (                  θ                  )                                            *                                                          ⁢                              ⅆ                θ                                                                        Equation        ⁢                                  ⁢        2.6            Similarly, by replacing R by T we get integrated transmittance. The sum of wavelength-dependent reflectance, transmittance and absorptivity equals 1 (Equation 2.7).R(λ)+T(λ)+A(λ)=1  Equation 2.7Reflecting MaterialsUV Reflecting Metal Sheets
Aluminum has been used for long time as a reflector of solar rays. Compared to other metal materials such as stainless steel and silver, aluminum is relatively inexpensive and can offer high reflectance (more than 90%) and good coverage of the solar spectrum. Among aluminum, stainless steel, and silver, aluminum displays the highest reflectance at the lower limit of the spectrum of solar UV radiation received at Earth's surface (˜290 nm). At this wavelength, stainless steel has 55% reflectance and silver has less than 10% reflectance.
However, the performance of aluminum metal deteriorates due to oxidation. Surface treatments such as anodizing and polyvinyl fluoride coating increase the useful life of reflective aluminum. Total reflectance includes specular and diffuse reflectance. Specular reflectance consists of radiation reflected at an angle which mirrors the incident light angle while diffuse reflectance consists of radiation reflected at other angles. At roughly 290 nm, unprotected aluminum loses around 15% of reflectance in 7 years, and in 14 years it loses around 60%. Protection of aluminum using anodization method slows down this reflectance loss. This protection caused around a 10% reduction in reflectance at low wavelength (near 300 nm) but shows an improvement in time of deterioration. A PVF protection showed a 20% less reflectance loss after 4 years compared with non PVF-protected Aluminum.
All-Polymeric UV Reflecting Film
In the 1970's and early 80's, several US companies developed an interest in research related to solar application materials. One of the promising reflecting materials that was patented by DOW Chemical was an All-Polymeric material. This is a plastic material that is inexpensive, light in weight, can resist harsh environmental stresses and can be designed for a specific wavelength reflectance to suit different purposes. This film is made of alternating thin layers of polyvinyl fluoride and polymethylmethacrylate. These materials have different refractive index (n) that promotes internal reflection. This material is a promising reflection material for solar UV application since it shows a good reflectance (around 90%) for radiation wavelengths of 300-400 nm for 1300 polymer layers.
Transmitting Materials
Potential materials for use in solar UV applications for transmittance are silica based materials such as fused silica and quartz due to good transmittance (>90%) of UVA and UVB wavelengths. Other materials such as transparent polystyrene and Polyethylene terephthalate (PET) used in SODIS (solar disinfection) bottles (0.45-0.55 mm thick wall) also transmit wavelengths in near-UV (Mani et al., 2006). Polystyrene has 20 to 40% transmittance between 290 and 310 nm, around 70% between 310 and 350 nm and 85% above. PET has roughly 80% transmittance above 320 nm but poor transmittance below 320 nm (McGuigan et al., 1998, Heaselgrave et al., 2006). Also, fluoroethylenepropylene (FEP) resins are used to make plastic tubes that offer variable UV transmittance (around 50% for 0.79 mm thick wall).
UV Dose-Response Relationship Studies
Dose-response relationships allow determination of sensitivity of microorganisms (inactivation rate) to UV. This relationship between dose and microbial response is generally obtained from bench-scale experiments in which well-defined UV doses are delivered to a microbial population. In most cases, the device used to deliver UV radiation to the microbial target is a shallow, well-mixed reactor under a collimated beam.
The UV dose-response behavior of microorganisms follows different kinetic models; there are microorganisms (for example bacteriophages) for which UV dose-response behavior is effectively described by single event (first order) kinetics, at least for a limited range of UV doses and inactivation responses. Others follow the Series-Event model, whereby a microorganism has to pass through a series of photochemical events before starting inactivation (Severin et al., 1983). The latter behavior presents a shoulder at the dose response curve. Pennell et al. (2008) developed the Phenotypic Persistence model in which the tailing behavior observed for some microorganisms was accounted for. This persistence was attributed to microbial phenotypic variation, or shielding of microorganisms by particles or microbial aggregates (Pennell et al., 2008).
Hijnen and Medema (2005) and Hijness et al. (2006) presented reviews of dose-response behavior for several relevant pathogenic microorganisms from numerous studies generated using low pressure lamps (254 nm). Data from dose-response relationships for two pathogenic viruses, adenovirus and poliovirus, were fitted using linear regression and the resulting kinetic coefficients of these two viruses were tabulated together with other common pathogenic viruses in drinking water. Of the viruses, a double stranded DNA adenovirus was reported to be relatively UV resistant as compared to other viruses; it has the smallest inactivation rate constant (k) of 0.024 cm2/mJ. Also in these reviews, dose-response curves for two common pathogenic bacteria, Campylobacter jejuni and E. coli O157 were fitted using linear regression; kinetic coefficients for these two bacteria and other common bacteria in drinking water are presented in FIG. 6. Among bacteria presented in FIG. 6, Legionella pneumophila is the most UV resistant (k=0.444 cm2/mJ) and Vibrio cholera is the least UV resistant (k=1.341 cm2/mJ). Presented information indicates that C. parvum (k=0.243 cm2/mJ) is slightly more UV resistant than Giardia (k=0.282 cm2/mJ). An important point is that the Hijnen and Medema (2005) and Hijness et al. (2006) reviews used linear fits for all microorganisms and accounted for the shoulder effect using an intercept on the response axis.
Solar UV Dose-Response Relationship Studies
Solar radiation, which is a polychromatic source with spatial and temporal variations, poses a challenge quantifying its effect on microorganisms with respect to these variations. Also, researchers have tried to simulate solar radiation using solar simulators but it is difficult to get a representation of solar radiation distribution around the globe or the region of interest. Others have used actual solar radiation to conduct experiments, and hence their results are specific for the area where the experiment took place. The following text will review findings from some studies that used ambient solar radiation and solar simulators to generate solar UV dose-response behavior for common pathogenic organisms found in drinking water.
Viruses and Bacteriophages
Heaselgrave et al. (2006) used a solar simulator to expose Poliovirus to a global solar irradiance of 850 W·m−2 (UVA and visible wavelengths from 320 to 700 nm) and found that a 4.3 log10 and 2.2 log10 inactivation was achievable at 1.2*106 mJ/cm2 and 6*105 mJ/cm2, respectively. For the conditions of their experiments, these doses required 4 and 2 hours of exposure, respectively.
Bacteria
SODIS has been proven to be effective for inactivation of bacteria. Berney et al. (2006) generated solar UV dose-response relationships for four types of bacteria, E. coli K-12 MG1655, Salmonella typhi, Shigella flexneri, and Vibrio cholerae. The exposure was performed using summer sunlight in Switzerland at UV wavelengths ranging from 350-400 nm, 37° C., for 6-7 hours exposure; these conditions correspond to a total UV dose of approximately 2400 kJ/m2 (2.4*105 mJ/cm2). The dose-response curve for E. coli shows the curve with a shoulder; E. coli demonstrated essentially no inactivation until a threshold dose of approximately 1200 kJ/m2 (1.2*106 mJ/cm2) was exceeded and 6.5 log10 inactivation was achieved after receiving a dose of 2.4*105 mJ/cm2. Shigella flexneri (lower left) required delivery of a threshold dose of approximately 400 kJ/m2 (4*104 mJ/cm2) before inactivation was observed, 6.5 log10 inactivation was achieved at a solar UV dose of 2400 kJ/m2. The dose response for Salmonella typhi fit with a linear regression. Relative to E. coli, S. thyphi were resistant to solar UV exposure; only 1.5 log10 inactivation was achieved at a dose of 2.4*106 mJ/cm2. Vibrio cholerae solar UV dose-response behavior was fit with linear regression and demonstrated 6.5 log10 inactivation at 900 kJ/m2 (9*104 mJ/cm2).
Since the radiation had a cutoff at 350 nm it does not accurately represent regions where solar UV radiation may be received at wavelengths down to 290 nm. For example, in an experiment conducted under Kenyan solar conditions (intensity of 106 mW·m−2), 5-6 log10 units reduction of Salmonella typhi was reached at a dose of 229 mJ/cm2 (6 hours exposure) and total non-infectivity at 8 hours (a dose of 305 mJ/cm2) (Smith et al., 2000, Berney et al., 2006).
Temperature is known to affect non thermo-tolerant microorganisms. Salmonella typhi, E. coli and Shigella flexneri were inactivated faster for temperatures above 50° C. and Vibrio cholerae is rapidly inactivated for temperatures above 40° C.
Protozoa
Two species of protozoa, Cryptosporidium parvum and Giardia lamblia, are of concern in drinking water. They have caused many fatal disease outbreaks and are resistant to chlorine. Due to Cryptosporidium's robust oocyst structure and the ability of Giardia to form cysts, both of these protozoan parasites can survive in water for a long time. McGuigan et al. (2006) exposed samples containing Cryptosporidium parvum oocysts and a surrogate of Giardia lamblia (Giardia muris) cysts to UV radiation from a solar simulator with a lower wavelength limit of 320 nm and an irradiance of 870 W·m−2 (UVA portion was estimated as 45 W/m2) at an ambient temperature of 40° C. Infectivity essays (performed using neonatal mice) of C. parvum showed non-infectivity after 10 hours of exposure, corresponding to a dose of 3*106 mJ/cm2 (1.6*106 mJ/cm2 UVA). Giardia muris cysts were rendered completely non-infective to mice after 4 hours exposure, corresponding to a dose 1.3*106 mJ/cm2 (6.5*10−4 mJ/cm2 UVA). The authors indicated that Giardia muris used in this study were believed to have higher UV resistance than G. lamblia, thus the solar UV dose-response of G. muris was assumed to represent a conservative estimate of the UV dose-response behavior of G. lamblia found commonly in water (McGuigan et al., 2006).